The Tb0.17Co0.83/Si multilayers prepared by a rf magnetron sputtering system with different Si thickness have been investigated. X-ray diffraction, magnetic measurement and Kerr rotation have been performed. With increasing thickness of Si layer tSi, the perpendicular anisotropy constant Ku decreased rapidly. The saturation magnetization Ms and the Kerr rotation θK decreased linearly when tSi increased. It was assumed that Co2Si and Tb had been formed in the interfacial zone between TbCo and Si layers. The reduction of Ku, Ms and θK is attributed to the decrease of the effective thickness of magnetic layer, which is linear with tSi.